1. Field of the Invention
The present invention relates to a vibration driven motor or actuator. More particularly, it relates to a vibration driven motor or actuator, which vibrates an elastic member, such as a bar-shaped vibrating member, by supplying electrical energy on electro-mechanical energy conversion elements provided to the bar-shaped elastic member. The vibration causes surface portions of the vibrating member to follow a circular or elliptic motion, thereby frictionally driving a movable member urged against the vibrating member. Such motor or actuator is especially suitable for optical equipment such as cameras and OA equipment such as printers.
2. Related Background Art
A bar-shaped vibration driven motor excites surface portions of a bar-shaped vibrating men%her to follow a circular or elliptic motion, thereby frictionally driving a movable member urged against the vibrating member.
Since the magnitude of the circular or elliptic motion is at most about several .mu.m, the vibrating member and a rotor must be finished to have a contact surface precision which is high enough to cover this magnitude, so as to maintain a contact state over the entire peripheral surface of the vibrating member. However, in practice, the vibrating member and the rotor cannot have such a high precision at a reasonable working cost. For this reason, spring characteristics are provided to the contact portion of the rotor so as to maintain the contact state over the entire peripheral surface of the vibrating member.
A vibration driven motor driven according to the above-mentioned driving principle has motor characteristics which exhibit a high torque at a low speed.
Therefore, since no gears are required for decelerating the motor, the vibration driven motor is utilized in products such as a camera, which must solve a gear noise problem, and requires quietness.
However, the vibration driven motor often generates noise during driving.
Such noise is caused by a non-driving vibration generated in the motor in addition to the vibration for driving the motor. The vibration is caused by a travelling wave.
The present applicant has proposed various countermeasures against this vibration. However, slight noise still remains.
FIG. 2 shows an observation result, using a microphone, of a vibration spectrum obtained when noise is generated during driving.
In FIG. 2, 1f is the driving frequency of the vibrating member, 2f is a frequency component twice the frequency 1f, and 4f is a frequency in an audible frequency range (20 kHz or lower), which generates noise.
These frequencies satisfy a relation 3f-1f=4f.
Since the driving frequency is the frequency of an AC signal supplied to generate a driving vibration, it can be shifted by .DELTA.f. If the shifted frequency is represented by f.sub.s, it changes to satisfy 1f.sub.s =1f+.DELTA.f, 2f.sub.s =2f+2.DELTA.f, . . . . On the other hand, the frequencies 4f and 3f change to satisfy the equations 4f.sub.s =4f-.DELTA.f and 3f.sub.s =3f, respectively.
Therefore, the vibration spectrum of noise corresponds to a differential frequency (4f.sub.s =3f-1f-.DELTA.f) between the frequency 3f and the driving frequency, and the frequency 3f can be considered as some natural vibration.
Upon examination of this natural vibration, it was found that the natural vibration was a bending vibration of a rotor main ring 2 shown in FIG. 3. FIG. 4 is a perspective view of the rotor main ring 2. In FIG. 4, a solid line represents the rotor main ring 2 before deformation, and a dotted line represents a bending vibration mode. A contact spring 2a (see FIG. 3) also suffers from a bending deformation to follow the main ring portion 2 although it is not shown in FIG. 4.
The object of the prevent invention is to present generation of such a vibration of the rotor main ring 2.
In addition, another noise may often be generated in addition to the above-mentioned noise.
FIG. 5 shows a vibration spectrum of this noise. Frequencies 1f, 2f, 3f, and 4f respectively represent the spectra of the driving vibration (1f), a vibration (2f) having a frequency twice the driving frequency, a natural vibration (3f) causing noise, and a vibration (.DELTA.f) of noise like in FIG. 2. In FIG. 5, however, these frequencies satisfy 4f=1f-(2f-3f).
Upon examination of the natural vibration 3f, it was found that the natural vibration was a bending vibration of the rotor contact spring 2a shown in FIG. 3. FIG. 6 shows the vibration mode of the contact spring 2a.
In this mode, the deformation of the rotor main ring 2 is very small, and only the contact spring portion 2a suffers from a bending vibration unlike in the bending mode shown in FIG. 4.